Black liquor recovery process and apparatus



Oat.n 3l, '1933.- E. G. GooDELL.

BLACK LIQUOR RECOVERY PROCESS AND APPARATUS Filed- March -28 19152 QS- @X 29.53 55m :Si

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NM @N M255, mi 6m Edward@ (fnde/ Patented oee 31,1933

UNITED sTATEs PATENT OFFICE BLACK LIQUon RECOVERY PnocEss APPARATUS AND 'I'his invention relates to a system of apparatus and process of recovering heat `and chemical values from the black liquor obtained in the manufacture of paper pulp by the soda-and sulfate processes.

More specifically this invention relates to a recovery system wherein the organic and vinorganic constituents of black liquorare reclaimed in a highly eicient manner and utilized by freeing the black liquor from its Water content in a spray' includes the digestion in a closed digester of. cellulose containing4 materials, such as chipped- Wood, in the presence of cooking agents' such as sodium hydroxide, sodium sulphide, sodium carbonate and the like whereby theA cellulose lor pulp is freed from lignins, resins and other no ncellulose materials.

After cooking in the digesters the contents are discharged into diffusers, wash tanks and the .like wherein the pulp is separated from the residual black liquors. These'black liquors contain over 50% of the weights of vdry wood cooked and contain the sodium compounds of the original cooking liquors.

Black liquor solids, on a dry basis, have a B. t. u. value of 6500-7000 B. t. u./pound. The economic value of an eiicient utilization of the heat content-and recovery of the chemicals in the black liquors 'is therefore obvious.

Processes proposed for recovering the chenilcals and utilizing theheat value of black liquor may, in general, be classed as follows:

1. The rotary incenerator process, which consists in evapora-ting in disc evaporators an'd the like a black liquor which has preferably been somewhat concentrated in steam evaporators. The concentrated llack liquor from the. disc evaporators is then pumped into Aa rotary drum or furnace where it is dried and partlyfburned to form black ash" by hot gases from a smelting furnace which are passed through the incenerator.

` -The black ash from the rotary incinerator is dropped into a lsmelting furnace preferably lof cated directly below the rotating incinerator and burned. The sodium compounds are fused off during the burning operation and recovered. The hot gases from the smelting furnace after passing '60 through the incenerator are used to generate steam.

2. The spray burning process, which consists in evaporating the black liquor to a concentration containing to 70% solids and spraying 65 the 4concentrated black liquor .directly into a smelting furnace wherein is eiected not only the evaporation of the black liquor but also the combustion of the organic solids and the fusion of the inorganic chemical content. The hot gases leaving the smelting furnace are used to generate. steam in a separate boiler unit.

y 3. The spray drying process, which consists in drying the black liquor in finely commlnuted form in a heated gaseous medium,preferably the linev gasesl resulting from combustion of the so sprayed dried black liquor powder in a smelting furnace. In this. type of process, as is more fully -described in my patents granted October 28, 1930 above referred to, a black liquor powder practicals0 ly moisture free is formed containing practically all of the organic and sodium compound content of the original black liquor.

In all ofthe processes referred to, thehot gases obtained in the smelting furnace are passed info 35 f a separate unit through interconnecting fiues and the like toa boiler unit Where they are used to generate steam. 4

I have now devised a process and apparatus wherein the smelting furnace andsteam boiler are one unit. My preferred process, as hereinafter described, includes the production of dried black liquor by a spray drying process but it 'is to be understood that my invention is not limited. to the use of a dried black liquor obtained by any particular method.

It is therefore an object of this invention to pro- Avidea single unit apparatus for ,the burning of dried black liquor, recovery of the sodium compounds contained thereinV and utilization of the heat of combustion for the generation of steam. It is another object of this invention to provide a process and apparatus whereby dried black liquor is burned directly below the boiler tubes for A the generationof steam from the heatcontent 105- of the dried black liquor with a minimum of radiation losses. It is a further object of this invention to provide a process and system of apparatus wherein black liquor is spray concentrated and the concen-v 110 black liquors.

As shown on the drawing:

The reference numeral 10 indicates acombined smelter furnace and boiler unit having primary boiler tubes l1, secondary boiler tubes 12, water and steam tanks or drums 13 and a mud vdrurn or tank 14. Steam generated in the boiler is circulated from the steam tanks 13 through a pipe line 15.

The boiler tube arrangement described is similar to that used in the Stirling type of boiler but it should be understood that other types of boiler tube arrangements may be used without departing from the scope of the invention.

Dried black liquor is partly burned in a bed 16 lat the bottom of the smelting furnace and the sodium compounds fused off through a taphole 17 into a dissolving tank 18 wherein they are prepared into cooking liquors for the digesters and reused in the preparation of pulp. A water' cooled blow pipe 19 supplies sufficient air to the fuel bed 16 to promote combustion of the dried black liquor. The air supply at this point is limited however to maintain reducing conditions so as to prevent any substantial oxidation of the sodium-sulphur compounds being fused out. If necessary additional air may be supplied through nozzles 2O located above the fuel bed zone. The introduction of air at this point creates a secondary combustion zone which insures complete oxidation and burning of the organic materials of the black liquor.

Dried black liquor powder or solids from any source are fed to the furnace through a spiral conveyor inlet 21 located above the fuel bed. In my preferred practice, as hereinafter more fully described, the black liquor powder is obtained.

from va spray drier and powder collector.

In the drawing the burning gases from the fuel bedpass around the boiler tubes 11, over a baille plate 22, then around the secondary boiler tubes 12, and under a baille` 23. At this point the gases are in a less turbulent state than when first leaving the fuel bed and the unburned black liquor powders carried along together with any fused sodium salts tend to drop down through a duct 24. It should be understood however that any type of steam boiler may be used and that the above description is specific to a particular type of boiler merely because such an installation has been in actual use. A fan 25 blows these solid particles into the powder collector or drier hereinafter described.

After passing between the boiler tubes the gases pass out of the smelter boiler furnace through a duct 26 into a spray drier 27. Concentrated black liquor having a solid content of about 50 to 55% is pumped from a storage tank 28 by a pump 29 through a pipe line 30 to an atomizer 3l by which atomizer it is sprayed in finely divided form to the` interior of the drying chamber 27. The flue gases entering through duct 26 effect substantial dehydration of the atomized black liquor. Part of the resulting dried powder falls into the spiral conveyor 2l at 32. `'ill'he balance of the dried Lacasse black liquor and all of the Hue gases (which are now somewhat moisture laden) through a duct 33 by an induced draft fan 34 driven through a pulley 35, or any source of motive power.

From the fan 34 the gas and powder mixture `is blown through a duct 36 into a powder separator 37 of the cyclone or other type. The dried black liquor powder is herein separated from the flue gases and falls into the spiral conveyor 21 at 38.

are drawn v Powder collected inthe spiral conveyor 21 at 32 l and 38 is fed'lnto the furnace 10 by any convenient method.

The flue gases from the powder collector 37 are next passed through a duct 39 into a scrubber 40 .wherein their remaining drying capacity is utilized to concentrate weak black liquor-to a sufllcient solid content so as to render it suitable for spraying into the drying chamber 27. The weak black liquor, which has preferably been somewhat concentrated in steam evaporators, is sprayed into scrubber 40 by atomizers 41. The sprayed liquor after passing through the scrubber 40 may be fed to the storage tank 28 through a pipe line 42.

The flue gases after passing through the scrubber 40 are substantially saturated with water due to their evaporative function in drier 27 and in the scrubber 40. These gases still have a sensible and latent heat content which is now reclaimed by means of a heat exchanger 43 into which the gases are directed from the scrubber. The function of the heat exchanger may be to heat feed water for the boiler 10, as a heater for black liquor or for any other purpose. After passing through the heat exchanger the spent flue gases are allowed to pass up a stack (not shown) to the atmosphere.

A comparison of my preferred process with the n `liquor proceed subsequently to the generation of steam by means of the heat content of the flue gases incident to the production of such steam. Further substantially all of the radiant heat of the burning solids is directly utilized for the generation of steam.

From the above description it will be seen that my process differs from heretofore proposed waste heat recoverymethods and results in .a highly efficient production of steam because:

l. A dried black liquor substance is .burned in the smelting furnace.

2. The dried black liquor substance is produced by a method which is not destructive to either the organic or inorganic content of the black liquor.

3. The construction of the smelter-boiler unit is such that the maximum heating value of the burning dried black liquor is absorbed by the boiler tubes.

4. The combustion of the dried black liquor powder will proceed without concurrent evaporation of moisture from the black liquor as in the spray burning process. This, of course, increases the production of steam.

5. The black liquor substance has not lost its organic or inorganic values through partial ycom claims refers to those alkali metal compounds dis-- v scrubber.

placein the In the claims below by the term .alkaline .treatment as used in connection with the manufacturing of pulp, I mean those processes known to the' paper art'in the digestionfof cellulose material as soda,.'sulphate, and modifications of these processes.

The term alkali metalvalues used in the solved` in the original cooking liquor such asvsodium hydroxide, sodium sulphide and the like.

I am aware thatmany changes may bey made and numerous details of my process may be varied through a wide range without departing from the principles of this invention, and I, therefore, do n'ot purpose limiting the patent granted hereon otherwise than necessitated by the prior.

91t- 'I j I-claim as my invention: 1. In the recovery of alkali metal values and `the utilization of heat content of blackQliquor produced in the manufacture of' pulp from cel' lulose bearing materials, the steps of introducing linto a smelting furnace the solids of black liquor in substantially dry condition, burning said solids .under reducing conditions, fusing off the alkali metal values, supplying additional air to the combustion 'gases'. directly above the fuel bed and completely burning. the-gases above the fuel bed directly under a steamlboiler the smelting furnace. i

2. In the process y.of utilizing the heat-value".`

gases. n l y ,'7. `A combined smelter and steamv generator for f in black liquor obtained in the manufacture. fof

paperpulp, the steps which comprise deliveringk to a smelting furnace-'boiler unit black liquorY solids in substantially dry condition, admitting a limited amount of air for partial'combustion close to the bottom of said smelting furnace, adding additional air to complete combustion at a point substantially above the top surface of the furnace bed and delivering the'hea'ted gas'es of said combustion to a steam boiler position directly above the smelting furnace Within the same unit.

3. In a boiler furnace for the recovery of alkali metal values and utilization of the-heat content of black liquors `1 obtained from the manufacture .of paper pulp, a fuel bed portion, a fluid cooled blow-pipe projecting into said portion, 'a feed forA dried vblack liquor located above the. fuel bed portion, air nozzles above said dried liquor feed, and

a steam boiler unit directly-above said furnace,`

l va single enclosure housing both-the steam boiler unit `and the furnace.

4. In the process of regenerating the active conthe flue gases,

unit mounted above stituents 'of black liquor, the steps which comprise burning substantially moisturev freev black 'liquor solids under reducing conditions and-substantially completely burning the lvolatilized andA gaseous.

5. 1in-the process vof recovering alkaline values from black liquor and efficientlyl utilizing the heat content of said liquor the 'steps whichv comprise introducing a dried black liquorfpowder'into a yboiler furnace'j'ust above a fuel bed composedof saidpowder lr'igja burning state, supplying-anl 1 amount yof air to said fuel bedsufhclent to main' tain combustion but not sufiicientto permit an ondary combustion zone by introducing airto the furnace above'the' point of entry of the dried black liquor and allowing the hot gases of combustion to arise into direct contact-with the boiler tubes within the furnace for the generation Aof steam.

6. A combined smelter and steam generator for the recovery of Aalkaline values and utilization of heat content in dried black liquor which comprises a housing defining an., upper combustion zone and a lower reducing zone,.boiler tubes in said upper zone, a. feed for black liquor powder near the top of said reducing zone','f:a blow pipe opening near the' bottom of said reducing zone, and air nozzles in` the furnace above said black liquor `powder feed line to furnish suilleientair for complete combustion of the `rising burning the recovery of alkaline valuesand efficient utilization of Vheat content in black liquor .which com# prises-a housing, a fuel bed zone atthe bottomfof said housing, an inclined base for said fuel bed; a tap hole for removal of molten alkaline Values in the lower end *of saidv base, a k,blow pipe in-isaid fuel bed for supplying 'a limited amount of rvair lthereto, aiscrew conveyor above said fuel bledv fory ,feeding black liquor solids directly thereto, air

nozzles above said screwv conveyor, primary and secondary boiler tubes substantially above said air nozzles'within said housing, a baille plate separating said primary tubes from said secondary tubes,.

. 95 oxidizingvatmosphere in said zone, ',creatinga sec. 

